1. Field of the Invention
The present invention relates to motor drive technology.
2. Description of the Related Art
In recent years, with the speeding up of personal computers and workstations, operating speeds of LSIs (Large Scale Integrated circuits) for processing in CPUs (Central Processor Units), DSPs (Digital Signal Processors), and the like, have been increasing.
In such LSIs, as the operating speeds, that is, clock frequencies, become higher, amounts of heat generated become large. There is a problem in that the generated heat from an LSI leads to thermorunaway in the LSI itself, or has an impact on neighboring circuits. Therefore, appropriate heat cooling of the LSIs has become a very important technology.
As an example of technology for cooling the LSI, there is a cooling method comprising an air-cooling system by a cooling fan. In this method, for example, the cooling fan is installed facing a surface of the LSI, and cool air is blown on the LSI surface by the cooling fan. When the LSI is cooled by this type of cooling fan, temperature in the vicinity of the LSI is monitored, and cooling level is adjusted by changing rotation of the fan in accordance with the temperature (Patent Documents 1 and 2). Furthermore, Patent Document 3 discloses technology in which a duty ratio of a pulse-width modulated signal is limited, so that a fan motor rotates at or above a predetermined minimum frequency.                Patent Document 1: Japanese Patent Application Laid Open No. H07-31190        Patent Document 2: Japanese Patent Application Laid Open No. 2001-284868        Patent Document 3: Japanese Patent Application Laid Open No. 2004-153955        
In the technology described in Patent Document 3, by using a three-input comparator to compare voltage (referred to below as temperature detection voltage) that depends on temperature generated using a thermistor, and voltage (referred to below as minimum frequency setting voltage) that prescribes a minimum frequency, with a cyclic voltage of a sawtooth waveform, a pulse-width modulation signal, whose duty ratio is limited, is generated.
However, in cases in which the three-input comparator is used, as the temperature detection voltage approaches the minimum frequency setting voltage, influence of an input offset voltage of the three-input comparator becomes significant. As a result, at a low temperature at which motor frequency drops, there has been a problem in that accuracy, that is, linearity, of rotation control of a fan motor deteriorates.